To determine whether magnitude and distribution of left ventricular hypertrophy is largely established at birth in patients with hypertrophic cardiomyopathy, or may substantially increase during the first years of life, 39 children with family history or morphologic evidence of hypertrophic cardiomyopathy were studied serially with echocardiography. Patients were initially investigated at ages 4-15 years (mean 11) and most recently at 9-20 years (mean 16). Over 2.5-6.8 year (mean 4) follow-up, 17 patients showed marked increase in magnitude and extent of pre-existent left ventricular hypertrophy, and 5 others demonstrated evolution from a morphologically normal appearing heart to substantial hypertrophy. In these 22 patients, increases in left ventricular wall thickness were striking (6-23mm; 101 plus/minus 62% change), greatly exceeded that expected to occur as a consequence of normal growth (13 plus/minus 10%; p less than 0.001) and were not associated with symptomatic deterioration or secondary to subaortic obstruction. These findings demonstrated that left ventricular hypertrophy may spontaneously develop or progress substantially in patients with hypertrophic cardiomyopathy during childhood when body growth is considerable. Since echocardiographic studies may be normal during childhood before the morphologic features of hypertrophic cardiomyopathy develop, echocardiographic screening of youthful relatives of patients with hypertrophic cardiomyopathy cannot always definitively exclude this disease.